Substantial recent data highlight the role of social functioning as a primary moderator of therapeutic response in individuals with post-traumatic stress disorder (PTSD) such that interpersonal difficulties and mistrust significantly and negatively impact treatment efficacy (Forbes et al., 2003; Forbes et al., 2005; Forbes et al., 2008). In addition, ample evidence suggests that psychotherapy improves the social and interpersonal lives of psychotherapy clients through improving clients' abilities to regulate their emotions and reduce social isolation (Yalom & Leszcz, 2005; Foy et al., 2000). While considerable research has been dedicated to exploring the neurobiology of emotional dysregulation associated with PTSD (Rauch, Shin, & Phelps, 2006; Etkin & Wager, 2007), and increasing data suggest that diverse psychotherapies affect neural functioning (Beauregard 2007), very little is understood about the neurobehavioral pathology underlying the debilitating interpersonal difficulties in PTSD, or the neurobiological mechanisms accompanying the improvements in social functioning that occur with efficacious therapy. Thus, the broad goals of this project are two-fold. First, we seek to examine the neural substrates associated with interpersonal dysfunction in PTSD using a social exchange game previously developed to assess interpersonal trust and cooperation in healthy (King-Casas et al., 2005) and psychiatric groups (Chiu et al., 2008; King-Casas et al., 2008). Second, we seek to examine changes in neurobehavioral substrates of social functioning following treatment for veterans with PTSD. Specifically, we propose to use functional magnetic resonance imaging (fMRI) and behavior within a well-characterized Trust Game to examine the neural substrates associated with interpersonal trust and cooperation in veterans with PTSD prior to and following empirically supported psychotherapy (cognitive processing therapy) for PTSD.